1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device capable of reducing manufacturing costs.
2. Discussion of the Related Art
As the information age has arrived, a display field for visually displaying an electrical information signal has been rapidly developed. Accordingly, various flat display devices with excellent performance such as slimness, light weight and low power consumption have been rapidly developed and have replaced Cathode Ray Tubes (CRTs).
Examples of such flat display devices include a Liquid Crystal Display (LCD), an Organic Light Emitting Display (OLED), an Electrophoretic Display (EPD; an Electric Paper Display), a Plasma Display Panel (PDP), a Field Emission Display (FED), an Electroluminescent Display (ELD), an Electro-Wetting Display (EWD), and the like, each of which includes a flat display panel for displaying an image as an essential component. The flat display panel includes a pair of substrates which are adhered to each other with a light emitting layer or a polarized material layer interposed therebetween.
Among such devices, the LCD device includes a liquid crystal panel including a lower substrate, an upper substrate and a liquid crystal layer filled between the lower substrate and the upper substrate. Since the liquid crystal panel is a non light emitting element, the LCD device displays an image using light incident to the liquid crystal panel and reflected or light supplied from a backlight unit provided on a rear surface of the liquid crystal panel.
The LCD device controls the light emitted from each of a plurality of pixels in frame units so as to display an image. In order to display a color image, the LCD device includes color filter layers each of which transmits a light of a wavelength region corresponding to any one of a plurality of colors including red (R), green (G) and blue (B) in correspondence with a plurality of subpixels.
Each of the plurality of pixels includes a first subpixel for emitting a light (hereinafter, referred to as a “red light”) of a wavelength region corresponding to R, a second subpixel for emitting a light (hereinafter, referred to as a “green light”) of a wavelength region corresponding to G, and a third subpixel for emitting light (hereinafter, referred to as a “blue light”) of a wavelength region corresponding to B, and displays a specific color by a combination of the red light, the green light and the blue light respectively emitted from the first to third subpixels. At this time, in correspondence with the first to third subpixels, the direction of the liquid crystal cell is changed by an electric field formed in the liquid crystal layer between pixel electrodes and common electrodes during one frame, thereby adjusting light transmittance. That is, the luminance of the light emitted from each subpixel (that is, light transmittance) may be adjusted by the intensity of the electric field formed in the liquid crystal layer between the pixel electrode and the common electrode.
FIG. 1 shows the respective voltage-transmittance curves of a red light, a green light, a blue light and a white light in a general LCD device.
As shown in FIG. 1, each of the red light Red, the green light Green, the blue light Blue and the white light White has light transmittance (denoted on a vertical axis of FIG. 1) which changes according to the voltage (denoted on a horizontal axis of FIG. 1) for determining the intensity of the electric field. In FIG. 1, the voltage-transmittance curve of the white light White is denoted by a dashed line, the voltage-transmittance curve of the red light Red is denoted by a dotted line, the voltage-transmittance curve of the green light Green is denoted by a solid line, and the voltage-transmittance curve of the blue light Blue is denoted by a dashed dotted line.
The color of the light emitted from each subpixel depends on a material configuring the color filter layer through which the light passes. That is, since the light emitted from each subpixel passes through the color filter layer including a material transmitting a light of a wavelength region corresponding to the color thereof, the color and the voltage-transmittance curve are changed according to the material of the color filter layer.
In the general LCD device, as shown in FIG. 1, the respective voltage-transmittance curves of the red light, the green light, the blue light and the white light have different forms. In detail, if the driving voltage is a first voltage (1), the red light has maximum transmittance, but the green light, the blue light and the white light have transmittances lower than the maximum transmittance. If the driving voltage is a second voltage (2), the blue light has maximum transmittance, but the red light, the green light and the white light have transmittances lower than the maximum transmittance. In addition, the green light has maximum transmittance at a voltage between the first voltage (1) and the second voltage (2).
As described above, using the color filter layer including different materials according to the color of the emitted light, the voltage-transmittance curve of the light emitted from each subpixel is changed according to colors. Since the voltage-transmittance curves of the red light, the green light and the white light are relatively similar to one another, a third voltage (3) for enabling the emission of the red light, the green light and the white light with relatively high transmittances is generally selected as the driving voltage. However, if the third voltage (3) is used as the driving voltage, the red light, the green light and the white light have relatively high transmittance, but the blue light has transmittance lower than those of the red light, the green light and the white light. Therefore, if the driving voltage is set to the third voltage (3), it is difficult to implement maximum luminance of the blue light. In addition, since the amount of emitted blue light is less than those of the other colors at the same voltage, the display of the pixel becomes inaccurate by the insufficient amount of the blue light and thus image quality deteriorates.
In the related art, subpixels for emitting the red light, the green light and the white light are driven by a driving circuit using the third voltage (3) as the driving voltage, and the subpixels for emitting the blue light are driven by a driving circuit using the second voltage (2) as the driving voltage, separately from the subpixels for emitting the red light, the green light and the white light. That is, since the driving circuit for driving the subpixels for emitting the blue light is included separately from the driving circuit for driving the subpixels for emitting the red light, the green light and the white light, the manufacturing costs of the LCD device are increased.